1. Field of the Invention
The present invention generally relates to image forming apparatuses which join a plurality of light beams together on a photoconductive drum where the light beams are modulated independently of each other by image signals, and particularly relates to an image forming apparatus which detects and corrects the position of the light beams where drawing starts
2. Description of the Related Art
Japanese Patent Application Publication No. 2000-187171 discloses a technology that achieves a compact-size optical scan system having a wide scanning span by controlling two drawing systems with one deflection means and by joining the two drawing systems together. This technology provides two sets of drawing systems, a single deflection means shared by the two drawing systems. Two beams are guided to different deflection surfaces of the deflection means for deflection in respective directions. The two beams are then directed to the same scan surface, thereby scanning the scan surface by dividing a single scan area on the scan surface into respective halves. It is also disclosed that the two scan beams are swept in opposite directions from a seam where the two scan lines join together toward the opposite ends.
Japanese Patent Application Publication No. 2000-267027 discloses an image forming apparatus based on an optical scanning unit that uses two beams simultaneously deflected by the same deflection means, and scans a scan area on the scan surface by dividing the scan area by half in the main scan direction.
According to this technology, a single polygon is used for the scanning of two drawing systems to start beam scans from around the center of an image, and joins the beams together in the main scan direction. This achieves a compact-size drawing system providing a wide span at a low cost. A crossing point detecting means (one dimensional CCD) for detecting a beam crossing position in the sub-scan direction is provided to detect the positional error of a scan line in the sub-scan direction caused by temperature variation (which occurs due to a minute displacement of a beam path attributable to the thermal expansion of the housing and/or the lens system). The error is then corrected to achieve satisfactory precision of a positional setting in the sub-scan direction, thereby suppressing an error in the sub-scan direction along the seam line.
Japanese Patent Application Publication No. 2000-187171 described above teaches a basic technology regarding an apparatus using two scan beams and a single deflection means. No measure, however, is taken to cope with a displacement in the main scan direction at a seam where the two scan beams join together.
Japanese Patent Application Publication No. 2000-267027 described above takes into account the correction of error in the sub-scan direction at a seam where two scan beams join together. No consideration, however, is given to the correction of error in the main scan direction.
Along the seam, error in the main scan direction has a detrimental effect on an image. An error of ½ dot, for example, will appear as a white streak in a halftone image. A dot pitch is 42.3 micrometers in a 600-dpi image, for example, so that a tolerable dot error would be about 21 micrometers. Since two optical systems join together, a tolerable error of each optical system is 10 micrometer, which is half of 21 micrometers.
Factors that cause error in the main scan direction include:    1) signal delay caused by temperature characteristics of a synchronization detecting sensor;    2) variation in magnification factors of lens systems caused by a temperature rise;    3) a change in a distance relative to the photoconductive surface due to a temperature rise of machinery;    4) a change in a distance relative to the photoconductive surface caused by eccentricity that appears during rotation of the photoconductive drum.
Errors in the main scan direction were measured to be 60 micrometers in the case of 1), 20 micrometers in the case of 2), 10 micrometers in the case of 3), and 70 micrometers in the case of 4) (when the eccentricity is 100 micrometers). In total, an error of 160 micrometers was generated.
The assignee of this application has already addressed the detection and correction of respective errors in the cases of 1) through 3). With respect to error in the case of 4), the assignee of this application has also proposed measuring the eccentricity of a photoconductive drum by mechanical means and correcting error based on the measured eccentricity.
There is a need for a new scheme that detects and corrects a positional error in the main scan direction at a low cost where such an error is caused by the eccentricity of a photoconductive drum along the seam where beams of the two optical systems join together.